l6712a STMicroelectronics, l6712a Datasheet - Page 11

• Manufacturer Part Number: l6712a
• Description: Two-phase Interleaved Dc/dc Controller
• Manufacturer: STMicroelectronics

CURRENT READING AND OVER CURRENT
The current flowing trough each phase is read using the voltage drop across the low side mosfets R
or across a sense resistor (R
transconductance ratio is issued by the external resistor Rg placed outside the chip between ISENx and
PGNDSx pins toward the reading points. The differential current reading rejects noise and allows to place
sensing element in different locations without affecting the measurement's accuracy. The current reading
circuitry reads the current during the time in which the low-side mosfet is on (OFF Time). During this time,
the reaction keeps the pin ISENx and PGNDSx at the same voltage while during the time in which the
reading circuitry is off, an internal clamp keeps these two pins at the same voltage sinking from the ISENx
pin the necessary current (Needed if low-side mosfet R
imum rating overcome on ISENx pin).
The proprietary current reading circuit allows a very precise and high bandwidth reading for both positive
and negative current. This circuit reproduces the current flowing through the sensing element using a high
speed Track & Hold transconductance amplifier. In particular, it reads the current during the second half
of the OFF time reducing noise injection into the device due to the mosfet turn-on (See fig. 4-left). Track
time must be at least 200ns to make proper reading of the delivered current.
This circuit sources a constant 50µA current from the PGNDSx pin: it must be connected through the Rg
resistor to the ground side of the sensing element (See Figure 4-right). The two current reading circuitries
use this pin as a reference keeping the ISENx pin to this voltage.
The current that flows in the ISENx pin is then given by the following equation:
Where R
ductance resistor used between ISENx and PGNDSx pins toward the reading points; I
carried by the relative phase. The current information reproduced internally is represented by the second
term of the previous equation as follow:
Since the current is read in differential mode, also negative current information is kept; this allow the de-
vice to check for dangerous returning current between the two phases assuring the complete equalization
between the phase's currents. From the current information of each phase, information about the total cur-
rent delivered (I
taken. I
current carried by the two phases.
Figure 4. Current reading timing (left) and circuit (right)
INFOX
SENSE
I
I
I
LS1
LS2
FB
is then compared to I
is an external sense resistor or the R
FB
= I
INFO1
I
ISENx
+I
SENSE
INFO2
Track & Hold
=
) and the average current for each phase (I
) in series to the LS mosfet and internally converted into a current. The
50µA
AVG
I
INFOx
to give the correction to the PWM output in order to equalize the
+
R
-------------------------------------------------
SENSE
=
R
-------------------------------------------------
SENSE
R
⋅
g
dsON
I
PHASEx
dsON
R
⋅
g
I
of the low side mosfet and R
PHASEx
sense is implemented to avoid absolute max-
PGNDSx
LGATEx
ISENx
=
50µA
+
I
INFOx
I
Rg
Rg
ISENx
50 µ A
AVG
= (I
PHASEx
INFO1
L6712A L6712
g
is the transcon-
+I
is the current
INFO2
)/2 ) is
11/27
dsON